1. Field of the Invention
Embodiments of the invention generally relate to implantable medical devices and more particularly to communication techniques to transmit data from the implanted medical device to an external programmer.
2. Description of the Related Art
Typically, implantable medical devices, in particular implantable stimulation devices, such as implantable therapy and/or monitoring devices including pacemakers, cardioverters and defibrillators or the like, may include data communication means to transmit data from the implantable stimulation device to an external device, such as a device external to the body, or vice versa.
Generally, a system for data communication with an implantable stimulation device thus may include an implantable stimulation device and an external device such as a programmer.
A typical implantable stimulation device includes a battery, a monitoring and/or therapy control unit, and in some cases one or more therapy units such as stimulation modules, and a memory for storing control program and/or data sensed by the implantable stimulation device. If the implantable stimulation device is a pacemaker or an implantable cardioverter/defibrillator (ICD), generally, the therapy units include stimulation (pacing) units for generating and delivering electric stimulation pulses to a patient's heart tissue (myocardium). Often, sensing units for sensing cardiac activity are provided. Sensing units may often process electrical signals that represent electrical potentials that may be picked up via electrodes, e.g., in the heart.
In order to transmit data acquired by the implantable stimulation device to an external device or to other implanted devices, generally, a telemetry unit may be provided. Typically, the telemetry unit may allow a bidirectional data communication, that is, the telemetry unit may transmit and receive data wirelessly.
Limited battery capacity of an implantable stimulation device often calls for energy-efficient data communication. An implantable stimulation device with limited battery power typically requires a low-power communication scheme in order to program it and to download acquired data. With extremely low-power communication, generally, more data can be transmitted more often.
Typically, the implantable stimulation device must source all of the energy required for transmitting data in all of the communication solutions disclosed in prior art. Pulses generated for data transmission generally use either current or voltage to create an electric field that is sensed at the receiver. Thus, the receiver often passively decodes the communication and the transmitter often uses sufficient energy to enable the receiver to sense the signal. This arrangement is often detrimental to extremely low-power devices, such as intracardiac leadless pacemakers (iLP). The available energy for these devices is generally extremely limited making high-energy pulses problematic. Typically, the pulses for data transmission must generally be sub-threshold because clinical complications could occur if communication caused capturing of surrounding tissue, e.g. myocardium of the heart. To reduce the possibility of capturing the heart, generally, very short transmission windows must be used to limit communication during times when the heart is refractory to stimulation. These reduced communication times often precipitate the use of high-data transmission rates that subsequently require higher clock rates for the implantable device. The higher clock rates generally complicate design and increase power consumption.
Typical communication schemes utilize sub-threshold pulses to galvanically transmit data to another device. When supra-threshold pulses are used, generally, the data is encoded within the pulse itself. The information in either case is often transmitted by pulses that are received and decoded. Typically, the electric field generated by the pulses is detected at the receiver, where all of the energy required for transmission is generated at the transmitter.
United States Patent Publication 2012/0078322 to Molin et al., entitled “Apparatus And Methods For Wireless Communication Via Electrical Pulses Conducted By The Interstitial Tissue Of The Body For An Active Implantable Medical Device”, discloses the use of biphasic pulses to maintain charge balancing while communicating.
U.S. Pat. No. 8,412,352 to Griswold et al., entitled “Communication Dipole For Implantable Medical Device”, discloses a device where the fixation mechanism is also connected to the communications module. The module of Griswold et al. uses electric pulses to communicate with another device.
U.S. Pat. No. 7,945,333 to Jacobson, entitled “Programmer For Biostimulator System”, discloses the combination of a programmer and an implantable device that communicate using encoded pulses. According to Jacobson, these pulses are described as modulated pacing pulses. Thus, if the data is encoded on pacing pulses then supra-threshold pulses are used.
Other typical communication schemes used for data communication by a telemetry unit involve either RF or magnetic communication. RF frequencies of ˜400 or ˜900 MHz or magnetic coupling in the 100s of kHz range generally require several mA of current to transmit and receive data. Such high current requirements are typically out of reach of devices with battery capacities of at most a few hundred mAh.
In addition, typical RF schemes require large antennas and magnetic coupling requires large transmit and receive coils for communication. Generally, the space available in an iLP (intracardiac leadless pacer), for instance, would not allow such large coils or antennas. iLPs are often designed to be placed within a heart chamber as opposite to conventional pacemakers, where the pacemaker itself is placed outside the heart and electrode leads extend from the pacemaker into the heart.
U.S. Pat. No. 6,704,602 to Berg et al., entitled “Implanted Medical Device/External Medical Instrument Communication Utilizing Surface Electrodes”, discloses a medical device communication system using sub-threshold pulses for electrical communication with external devices. The medical device communication system of Berg et al. includes an implantable medical device and external devices. The external devices may be connected to the skin of a body with a plurality of electrodes. The implantable medical device includes stimulation electrodes, surface electrodes in contact with tissue of a patient, and a can including a pulse generation circuit. The reference also discloses an electrode switching circuit coupled to the pulse generation circuit and serves to deliver electrical stimulation pulses to the stimulation electrodes as therapy to a patient. Furthermore, Berg et al. discloses wherein the electrode switching circuit also serves to deliver subthreshold pulses to the surface electrodes of the can in a predetermined pattern of modulations constituting an encoded data signal that propagates as a signal transmission through the patient tissue. According to Berg et al., the plurality of electrodes connected to the external device serve to receive the sub-threshold pulses and allow the external device to detect the encoded data signal.
For example, United States Patent Publication 2012/0109236 to Jacobson et al., entitled “Leadless Cardiac Pacemaker With Conducted Communication”, discloses a system for pacing a heart of a human including a leadless pacemaker in a hermetic housing with at least two electrodes and at least one external device with at least two skin electrodes. The electrodes of Jacobson et al. appear to deliver energy to stimulate a heart and to transfer information to or from the skin electrodes of the external devices. The information in Jacobson et al. is preferably encoded in sub-threshold pulses delivered by the electrodes and generated by a pulse generator in the housing of the leadless pacemaker. According to Jacobson et al., the hermetic housing of the leadless pacemaker may further comprise a controller configured to communicate with the external devices by transferring information through the electrodes. Jacobson et al. also discloses wherein the controller may be configured to communicate with the external devices outside of a refractory period or pacing pulse.
In view of the above, there is a need for a communication scheme with an implantable device that does not employ RF or magnetic coupling.